Solar power generation, which converts light energy such as sunlight to electric energy is an extremely clean way to generate electricity because it is not accompanied with CO2 emission, and has been expected solve the global warming issues by reducing greenhouse gas emissions. An organic thin film solar cell can be expected to be large in area, and produced by a simple and inexpensive manner and thus is deemed a promising next generation solar cell because of its light weight and great flexibility. However, a significant increase in the conversion efficiency of the organic thin film solar cell has become an important issue towards the practical use of the cell.
In 1992 Sariciftci demonstrated that a hetero junction cell of an electrically conductive polymer that is a hole transport material and fullerene C60 can provide an efficient charge separation (see Non-Patent Literature 1, Patent Literature 1).
Furthermore, for the purpose of enhancing the compatibility of a fullerene with a hole transport material, a methanofullerene was synthesized, wherein phenyl and butyric acid ester group are cross-linked via methylene (phenyl-C61-butyric acid methyl ester: PCBM). When poly[2-methoxy,5-(2′-ethyl-hexyloxy)-p-penylene vinylene] (MEH-PPV) wherein an alkoxy group is introduced in poly p-phenylene vinylene is mixed with PCBM to form an active layer, the resulting MEH-PPV/PCBM system with this active layer was significantly improved in photo conversion efficiency as compared with C60 (see Non-Patent Literature 2).
Sariciftci et al has accomplished an energy conversion efficiency of 3.5 percent with a bulk-hetero junction structure comprising a mixed active layer of a conjugated polymer such as poly(3-hexylthiophene), P3HT that is a hole transport material and a methanofullerene derivative PCBM (see Non-Patent Literature 3).
Heeger and Carroll et al each have reported that an energy conversion efficiency of approximately 5 percent was accomplished by heating this blend film device (see Non-Patent Literatures 4 and 5).
In order to promote an significant improvement in energy conversion efficiency, it is now important to develop novel p-type semiconductor materials and n-type semiconductor materials. P3HT and PCBM are now standard materials for a p-type semiconductor and an n-type conductor, respectively to manufacture a device, and thus the conversion efficiency has been currently enhanced only by improving the device. Although it has been reported that some new polymers are useful for the p-type semiconductor, there are only few examples of development of fullerene derivatives for the n-type semiconductor used in a photoelectric conversion material, and thus it is no exaggeration to say that there is nothing to beyond PCBM.
When a consideration is now given to the design guideline of a fullerene derivative, the compatibility of the fullerene derivative with a hole transport material is extremely important as apparent from the fact that PCBM was synthesized for the purpose of enhancing the compatibility of a fullerene with a hole transport material to produce a donor/acceptor complex film, and the MEH-PPV/PCBM system was significantly improved in photoelectric conversion efficiency as compared with C60. It has been also reported that substitution of the phenyl ring of PCBM with an electron-donating substituent increases LUMO (lowest unoccupied molecular orbital, the orbital with the lowest energy among the molecular orbital unoccupied with electrons), and when the PCBM with the substituent is fabricated into a device, it can increase the open end voltage, resulting in an increase in energy conversion efficiency (Non-Patent Literature 6).
Under these circumstances, the molecular design of fullerene derivatives has been demanded, which are excellent in dissolubility, enhanced in compatibility with polymers that are p-type semiconductors, and have a high open end voltage.
The inventors of the present invention proposed a novel methanofullerene derivative to satisfy these demands (Patent Literature 2) but it is not sufficient in open end voltage. A methanofullerene derivative with a higher open end voltage has been demanded.